Getting the right cable sizes for your system

We have now talked about most of the aspects of a typical solar system installation, but there is one aspect we haven’t touched upon yet.  Determining cable sizes is another very critical thing that you have to work into the design of you system to not only make sure that the efficiency of your system is optimal, but to also make sure that you don’t run any risk of cables melting or burning because they  are too thin for the job.  The converse is also true – you don’t necessarily want to “over-spec” the cables that you acquire too much; copper cabling is pretty expensive these days and you want to make sure that you buy just the right cables for the specific application, including off course a little bit of a buffer for safety and possible future expansion.

Things to consider when determining cable sizes

There are really four important things you have to be concerned about when thinking about getting the correct cables to connect everything in your system.  They are:

  • Current carrying capacity of the cable
  • The average ambient temperature that your cable will be working in
  • Voltage drop over the length of the cable
  • The load factor of the specific load that you are going to use the cable for

The first thing, and what I consider to be the most important thing, is the current carrying capacity of the cable.  This is a number that is specified in ampere (A).  You should always stay well within the limits of the current carrying capacity of the cable, no matter which part of your system it is connecting.  This also goes for the cables you use to do earth connections!


OK, to see if the cable has enough current carrying capacity, you simply have to determine what the voltage at the feed or source side of the cable is, what the approximate length and ambient temperature of the cable will be, and then also what the load is on the other side.  Let’s use an example of a 230 volt supply, and let’s say that you have a 1 kilowatt load that you want to drive on the other end of a 35 meter cable, operating at 35 degrees Celsius (86 degrees Fahrenheit).

From the basic principles that we discussed before, you will remember that Ohm’s law says that     P = I x V, (there are other aspects/variations of Ohm’s law as well but don’t fret over that now!) which simply means that the power (P, or wattage) is equal to the current (I, measured in amps) times the voltage (V, measured in volts).  We have the value for the voltage, which is 230 volts, and we also have the size of the load, which in the case of our example is 1 kilowatt, or 1000 watt.  From the equation above, we can then figure out the current, I = P / V, (trust me, the formula is manipulated to reflect this with a little basic math!) or rather, the current (I, in amps) that the load will draw through the cable is equal to the power (P = 1000 watt) divided by the voltage (V = 230 volt).  Just hold on tight here, it’s actually really simple – that means that I (the current) is equal to 1000/230 = 4.35 amps.  See, really not that difficult at all!

In this example, it means that we have to provide a cable to connect this load of 1000 watt to the supply of 230 volt, which will be able to carry at least 4.35 amps, over the 35 meter length of the cable.  It is good practice to add about 20% additional capacity to the cable size.  To do the math, you can either add 20% to the load (20% of 1000 watt is 200 watt, added to the load gives a total of 1200 watt.  I = 1200 / 230 = 5.22 amps), OR you can just add it to the minimum current carrying capacity of the cable that you choose (4.35 amps x 1.2 for the additional 20% = 5.22 amps).  Including a 20% “buffer” or safety factor, means that we have to choose a cable that has a minimum current carrying capacity of 5.22 amps.  [Not finished yet, we still have to do a few other checks regarding the cable size, which I discuss below!]

From Table 1 below, we now have to choose the cable that is closest to the current carrying capacity that we need.  The cable with the specification closest to what we need is the first one, with current carrying capacity of 11 amp, and size (thickness of the copper conductors) of 1 square millimeter (the British measures are also indicated in the table for convenience, in this case it is 0.0015 square inches per conductor, also with a current carrying capacity of 11 amps).


Ambient temperature also plays a role.  (This is the second factor you need to consider).  Table 2 below gives the “temperature factor” for a range of temperatures.  Choose the average temperature in your area that is closest to the next highest temperature setting listed in the table.  If you look at Table 1, you will notice that all ratings are given for the cable at 30 degrees Celsius (86 degrees Fahrenheit).  To compensate for the cable operating at a temperature of 35 degrees Celsius, not 30 degrees, we have to multiply the current carrying capacity of the cable that we chose, by the Temperature Factor.  In this case, from Table 2, the temperature factor is 0.97.  If we multiply the current carrying capacity (11 amp) by 0.97, we get 10.6 amp.  Because our requirement is only 5.22 amp, and the temperature compensated current carrying capacity of the cable is 10.6 amp, our cable is clearly still good enough according to the math up to here.


The third thing we have to check to make sure that our cable will be fine for this application, is to make sure that the voltage drop over the length of the cable is also acceptable.  The accepted norm is that the maximum allowable voltage drop from the connection point at the source of electricity, to the furthest end of the cable where you connect the load should not exceed 2.5% (Just for interest sake: This proposed limit for voltage drop over cable lengths comes from the IEEE rule B-23, which is a set of standards for just about everything electrical, managed by the IEEE – Institute for Electrical & Electronics Engineers.  The maximum allowable voltage drop for a main feeder cable from the “meter box/point of supply” to the “distribution box” is 1.25%).  In our example we are working with a 230 volt system supply, and we assume that the load will be the furthest end of the system, which means that the maximum allowable voltage drop will be 2.5% of 230 volt, 2.5% x 230 = 5.75 volt.  So, 5.75 volt is the maximum allowable voltage drop for this specific cable in this specific application.

From Table 3 below, we can see that the voltage drop for our chosen cable is 41 millivolt per amp-meter.  (Table 4 is given for your convenience if you live in a part of the world where the imperial system is used for measurement – The exact same principles apply for all calculations).  This voltage drop of 41 millivolt per meter, is given as a standard for 1 ampere of current flowing through the cable over a 1 meter length.  Now, from the calculation we did above, you can see that we will not be drawing only 1 ampere through the cable, but actually, we will be drawing 5.22 amps through the cable (including off course our “safety factor” of 20%).  We therefore need to multiply the 41 millivolt by 5.22 to figure out the real voltage drop we will get per meter, with our load of 1000 watt connected at the end of the cable.  Hope this makes it clear!  With the specification in the British/Imperial system, this calculation works exactly the same way.  The numbers differ slightly between the metric system and the imperial system because the cable thicknesses are slightly different.

So, based on the above, to calculate the voltage drop, in the case of our example above, over the length of the cable (35 meters), we need to multiply the specified voltage drop in millivolt by the maximum current carrying capacity of the cable and by the length of the cable.



That means that the total voltage drop in our chosen cable over the 35 meter length will be (41/1000) x 11 x 35 = 15.8 volt.  Now, before you rush off to go and buy your cable, there is the last factor that you have to take into account.  This is the load factor – it is a measure of the current that we are actually going to draw through the cable, in relation to the maximum current carrying capacity of the cable that we chose.  So, let’s now do the calculation for the load factor as well.

The load factor is simply the current we expect to draw through the cable (in the case of our example this value is 5.22 amps), divided by the maximum temperature adjusted current carrying capacity of the cable.  The load factor will therefore be 5.22/10.6 = 0.49.  We now need to multiply the voltage drop by the load factor to determine the ACTUAL voltage drop that we will get in this specific installation.  This calculation is: Voltage drop (actual) = calculated maximum voltage drop (15.8 volt) x Load factor (0.49) = 7.74 volt.  You can clearly see that this ACTUAL voltage drop from our calculation is more than the maximum allowable voltage drop specified earlier on (5.75 volt).  This means that we have to choose the next “thicker” cable and do the calculations again to make sure that this time it will all work out.

Because our new choice of cable size will be thicker, our current carrying capacity will be higher, and we therefore know that our previous calculations of the minimum size of the required cable according to the current carrying capacity will still be valid.  We do still have to re-do the temperature compensation calculation, as well as the voltage drop calculations, and the load factor calculations to make sure that the next size cable will be suitable for the application.  The voltage drop in this next “thicker” size cable, according to Table 3 above is 28 millivolt, at a current rating of 13 amp  [This is the data from the second line in the Table, 13 amps, 28 millivolts, and cable size of 1.5 square millimeters].  The temperature compensated maximum current carrying capacity of this new cable will be 0.97 x 13 amp = 12.6 amp.  Our calculation will then be: Voltage drop = (28/1000) x 12.6 x 35 = 12.35 volt.  Now we need to bring the load factor into play again.  Our new load factor calculation will be ACTUAL current (5.22 amp) / maximum current carrying capacity (12.6 amp) = 0.41.  The calculation will then be Voltage drop (ACTUAL) = 12.7 volt x Load factor (0.41) = 5.21 volt.  The maximum allowable voltage drop that we discussed before is 5.75 volt, which means that our cable is now within all allowable requirements – the cable is “thick” enough to handle the load, and to also deal with the voltage drop, as well as the elevated ambient temperature.  The cable you have to use for this application is therefore a 1.5 square millimeter cable, with the specifications indicated in Table 3 above.  The cables specified in the tables are all pretty standard right across the world from a sizing perspective.  If all else fails guys, rather buy a slightly thicker cable than what your calculations tell you  to buy – that is always the safer route to follow.

Even to do the calculations for an entire electrical system, you simply follow the same principles to calculate the “size” of the cable required for each branch of each circuit.  When you do the design of cable sizes for internal wiring in a home, factory or other building, you also have to be aware of the minimum specifications that are laid down by local or national authorities in the country or local area where you live.  These regulations may indicate “thicker” cable for such internal wiring applications, which is essentially set up so that there is always a higher “safety factor” in these systems to keep all of us safe.

In a future post I am planning to put a whole system together and do all the necessary calculations to show you how the whole lot is put together.

Simplified summary

This is a very short summary of how to determine the size or gauge of the cables that you will need to connect everything in your solar system:

  • Determine the source voltage
  • Determine the size of the load at the end of the cable
  • Measure the length of the cable and figure out the average ambient temperature at which this cable will be used
  • From the table get the relevant data for your cable that you chose
  • Do the calculations as discussed above
  • You are now ready to go out and buy the most appropriate size of cable that you will need for the installation of your solar system
  • Enjoy and have fun
  • Always remember, if you have any doubts, questions or concerns o CONTACT A PROFESSIONAL IN YOUR AREA AND ASK FOR ADVICE BEFORE YOU DO ANYTHING! (This will always be my  little “mantra” for everyone – please, always take care, make use of the safety instructions given in a previous post, and always, always check and ask for a professional’s input if you have any doubts)

That’s it for this week…

Once again, if anything is not clear to you, or if you have any specific questions regarding the discussion this week, please reply to this email and I will gladly assist you to figure it out.  Until next week, when we will have a look at an entire system and all the requirements and issues involved.  I may have to split that discussion over more than one post to make sure you don’t get bored in the process.  We will take it as it comes.  Happy calculating!

The right kind of protection and switching for your solar system

I am sure that you will agree that the value of the solar system that you have acquired warrants some serious thought regarding the best way to protect it against nasty surprises like a lightning strike or some other electrical surges or faults.

Lightning protection

In terms of protection against lightning, a good earth will go a long way to protect your system, together with some form of lightning protection.  To install a good earth for your system is pretty straight forward.  You can either use an earth rod (a copper rod of anywhere between 1m and 2.4m long, or you can create an “earth mat” buried underground.  Earth mats are a pain and therefore I suggest you go for the earth rod – it is just much easier to install and the difference in efficiency between an earth rod and an earth mat is debatable.  If you really wanted to know, an earth mat is a long length of copper wire, or a “mat” of copper wire that you made yourself, which is simply buried about 0.5 to 0.75 meters under the ground.  You then simply connect you earth wire from your system to this earth mat.  An earth spike on the contrary is simply a copper spike that you can get at any electrical supplier, which you drive into the ground with a hammer.  You then do the connection of your system earth to this earth spike.  Here is a picture of my specific installation.  You can only barely see the top of the earth spike, where the earth lead is connected which goes to the solar equipment.


Connect your system earth cable to the spike, and also connect steel or aluminium casings to the earth spike.  If you can keep the area around the earth spike moist, this will significantly add to the quality of the earth connection that is made by the spike.  Think about driving the spike into the ground close to where you may be watering the garden – that way it will always have some moisture around it, adding to a good earth.  Make sure that you use the correct cable size when earthing – we will talk about this issue in a future post [Determining cable sizes for your system].  Try as far as you can to have one point somewhere in your system housing where you connect all earth cables together, and then take on considerably thicker cable to the earth spike.  Also – don’t connect more than one point on the equipment casing to earth!  This may lead to what is called “ground loops” which can cause endless headaches where you have all sorts of spurious electrical activity that you can’t explain with any math!   Identify one point where you can connect the whole lot, and then connect one much thicker cable to the earth spike.  It just removes a lot of potential challenges from your system stability if you follow this advice.  If you want to use more than one earth spike, make sure that you securely connect all earth spikes to each other, and position them in close proximity so that these interconnections will be a short as possible.  This way the earth spikes “look like” one solid earth point to the system.  You want to avoid installing one earth spike on one side of the installation, while you install another on the other side, making earth connections to both.  This will most likely lead to those dreaded “ground loops” that I was referring to earlier on.

Try to keep the length of earth cables as short as possible, and make sure you use really good quality lugs, connectors and screws when you do the earth connections.  I would suggest that you spend a little more money here on the connecting nuts and bolts for example, and rather buy stainless steel or copper bolts and nuts to do the earth connections.  The normal steel bolts and nuts that we use for everything else tend to rust or gather corrosive build-up over time.  I am sure you will agree that you are not going to “inspect” your earth connections regularly, and this means that your steel bolt and nut connections may deteriorate to such an extent that they are no longer effective, without you knowing about it.  So, for extra peace of mind, use stainless steel or copper bolts and nuts for the earth connections of your system.

Please understand that we can only do what would reasonably be expected in terms of protecting our system against lightning.  Lightning is really unpredictable and can strike literally anywhere, and many times with immense amount of energy contained in them.  If you experience a direct strike to your system, there is very little that you can do that will prevent damage in such a situation.  Hopefully most of the lightning strikes that you will experience will not be direct strikes, and therefore the protection that we can put in place will make a difference.  The bad news is that unless it is a destructive type of lightning protection unit that you install, you will never know whether the protective devices that you did install made any difference.  It does however give quite a bit of peace of mind knowing that you’ve done everything reasonable to protect your system.  The same kind of argument goes for surges experienced in an electrical network, especially if you are grid connected.  You never know what nasty surges can come down the line towards your system.  There are however inherent “protective devices”, or devices with protective characteristics installed in a typical electrical network all over the show.  Here I am thinking specifically of transformers that feed electricity into your home.  They have an inherent characteristic of “arresting” or “softening” surges and lightning strikes.  Capacitors installed on distribution lines for balancing purposes have a similar effect.  So, the bottom line is, don’t lose sleep over it, just do what you can.

Switching and fusing

From a safety or safe operating perspective, there are also some basic aspects that you have to take into account.  To make sure our system is safe for maintenance after installation, as well as for periodic inspection, we usually use switches to safely isolate certain parts of the system.  This allows you to safely do maintenance or inspections knowing that the part of the system you are working on is not energised.  Basic switching required for your solar system will include installing switches (in electrical terms normally called “circuit breakers” or “isolators”) between your solar panel array and the MPPT; between the MPPT and the inverter/charger; between the inverter and the battery bank; between the inverter and the load you are energising (your home electrical system).  This is the absolute minimum switching that you should consider installing right from the start.  It is also important to note that each “circuit breaker” has a specific current rating, above which it will automatically interrupt the circuit.  This is yet another form of over-current protection.  A typical installation with the switching as suggested above, will look like this:



This is crucially important!  Please always make sure that you “lock out” the part of the system that you are working on.  Put some simple procedures in place for yourself, making sure, and checking at least twice that the specific part of the system you will work on is isolated.  I am talking of first-hand experience here – having worked in this industry for 20+ years, I can assure you that you don’t want to go through the experience of realising that the system you are working on is actually live!  Always, always make sure that you have done the necessary measurements with a simple piece of equipment like a true RMS multimeter, ensuring that everything that needs to be switched off is actually switched off!  You simply use a multimeter, set it to the alternating current setting, making sure that you have selected a range of voltage measurement that is appropriate for (higher than the voltage you want to measure…) the voltage you expect to measure on your system.  You then measure across the “neutral” and “live” wires to see if there is any voltage present.

You want a reading that is zero before attempting any work on that part of the system.  [The real technical term here is that you are measuring for a “potential difference” between the two conductors, just for those of you who are interested…].  The next logical step is to “lock out” the switches that you have switched off in order for this “state” to continue for as long as you are working on that part of the system.  Please don’t take this “locking out” notion lightly – use a physical lock to make sure nobody can accidentally switch on anything and that way energise the system while you are working on it.  Again, I am talking from first-hand experience here!  It has happened to me and you don’t have to experience it to believe me!  In the mining industry we even have certain documentation that has to be completed on a daily basis indicating where work will be done and which parts of the system will be de-energised for what periods of time.

The message here is: DON’T TAKE ANY CHANCES! Always be safe and double check all systems before you start doing what you planned on doing.  There are just too many recorded incidents of mishaps that took place and that either injured or killed people because these simple steps were not followed.  And another thing – don’t be fooled by the supposed “low voltage” [ie. 12 volt, 24 volt or 48 volt] of your solar system.  It is not the voltage that kills, it is the current (the “amps”), and apart from that, there is just as much “energy” present on the “low voltage” side of a system [off course excluding losses!] as there is on the “high voltage” side (110 volts in the USA or 220 to 250 volts in Europe and other places).  Some research indicates that you could potentially receive a fatal electrical shock from as little as 75 milli-ampere (yes, that is milli-ampere, meaning 75 thousandths of one ampere!!).  The bottom line is, it is YOUR responsibility to do all necessary checks before attempting any work on a system – nobody is going to take the blame for you on this one if you screw it up!  If ever you are in doubt as to how to do it or how to make sure, consult a professional in your area – it will be the best money you ever spent!

The logic and convention of working on an electrical system

Here are just a few thoughts on convention in terms of how you should work on an electrical system.  Apart from making sure that you always work on a system that is not energised, there is some logic to follow in terms of how you put together your solar system.  When doing a typical solar system installation, always start making connections from the “dead” end of the system; in other words start connecting cables from the inverter to the MPPT, and from the MPPT to the switches on your distribution panel (obviously with the switches or “circuit breakers” as they are called in the “off” position).  Once you have all connections made, only then proceed to start connecting the “live” or the “hot” parts of the system to the rest of the system.  It is also a good idea to install all your circuit breakers or switches and your fuses in a “distribution box”.  This is a sealed box made specifically for the installation of electrical switches and equipment.  You only need a small box for all the bits and pieces that you will install on your solar system.  This is a typical installation done in such a box:


Lightning protection

The basic premise of lightning protection devices is that they will “arrest” or stop the surge of lightning (the electrical energy that is caused by a lightning strike) before it has the chance to energise your system with the immense energy that it normally contains.  It does this by providing a low resistance, short route for this energy to follow to discharge into the earth.  These devices are normally installed from the neutral and lives wires of your system (or the “hot” and “cold” feed wires if you live in the USA) to earth.  The theory is that when a huge surge of electrical energy does arrive at this device (a lightning strike is really just a huge surge of electrical energy), it will “arrest” the energy by means of creating the shortest route possible from where it is installed to earth.  That is the reason why these devices are installed from the live or neutral wires to earth.  These devices come in many shapes and sizes.  The best you can do is to find out from a local electrical equipment supplier what the minimum lightning arrestor devices are that should be installed in an electrical network or system.  A local electrical contractor that is worth his or her salt, should also know what the typical installation regarding lightning protective devices is that is appropriate for your area.  There are areas where lightning strikes are particularly prevalent, and in such cases, we tend to install more than the usual amount or size of lightning protection.  This is why I am suggesting that you speak to a local practitioner in your area.  [See the picture above for the installation of these lightning protection devices – they are the ones second and third from the left in the “distribution box”]

I have installed one device on each of the live and neutral wires of my system where it comes out of the inverter and feeds into the house.  Whether this is the best position for these devices to be installed is also debatable, but this is all I really need in my area.  We don’t experience a heck of a lot of lightning in our area, therefore these should suffice for the unlikely event of us getting a strike close by.


Fuses are used for over current protection in a system.  Remember that any electrical system, including the one you are designing for your home, is designed for a specific load and this implies that there will be a maximum amount of current (“amps”) that will flow through the system.  In the event of a “short circuit” happening in your system somewhere, the current flowing through the system will dramatically increase with the possibility of frying just about everything that happens to be in its way.  You therefore want to prevent the current from ever going over a specified maximum number of “amps” or “amperes” (Remember that electrical current is measured in “amps” or “amperes”).

For this reason, fuses are installed in line with at least one of either the live or the neutral wires feeding from the battery bank to the inverter.  The convention is to install a fuse of a specific rating in line with the “live” wire.  If you look at the picture I included above, the fuse is the item that is installed on the very far left of the “distribution box”.  If you install one of these in your system, you should be well protected.

Earth Leakage protection

This kind of unit is used to protect an electrical system from current “leaking” from the live (“hot”) or the neutral (“cold”) wires in the system.  So, when there is for example a break or damage to the insulation of one of these wires anywhere in the system, and this causes a small current to leak to earth, the earth leakage unit (ELU) will “trip”, or rather automatically interrupt the entire installation.  An earth leakage unit is normally installed in line with the main feed into your home, which means that it will detect any leakage of current to earth, and it will then interrupt the whole lot!  This off course leads to quite a bit of frustration when it comes to fault finding – all you know when the ELU unit has “tripped” is that you have an earth fault somewhere… A process of elimination is then used to isolate the specific circuit which is causing the fault.  Further iterations of this process then have to be done to determine exactly what and where is causing the earth fault on that specific circuit that was isolated as the cause.

Bottom line is that this kind of “earth leakage” protection is very important to protect us as users of the electrical system from accidentally touching some piece of equipment or metal enclosure somewhere which happens to be the cause of the leaked current.  Because we touch the piece of equipment or enclosure, we will then form the closest path to earth (especially if you are barefoot!) for the electrical current, which means that we stand a chance of getting an electrical shock.  It is at this moment, when the “leaked” current is detected by the ELU and the entire system is interrupted, or shut off.  Yet another word of caution here – whenever the ELU does interrupt the electrical supply (ie. It “trips” or automatically switches off the main supply), there is a reason for it.  Look for the cause of the fault until you find it.  It would be irresponsible (and illegal!) to bypass a circuit to ensure that the ELU doesn’t interrupt the system.  The ELU also protects against most instances of fire caused by leaked currents.

That’s about it for the basics of protection, switching and safe working on your solar system.  Just a last note again – whenever in doubt about anything, please consult a professional!

Simplified summary

This is a very short summary of how to best protect your solar system, while making sure that you always work safely:

  • Install a proper earth spike and make good quality connections with the appropriate thickness (“gauge”) of wire or cable. This is your first line of defence specifically against lightning strikes and spurious “spikes”
  • Install the minimum switching as discussed between the different components to enable easy maintenance and safe working after you have finished the installation
  • Apply the simple rules suggested to always be safe when working on the system – Remember to contact a professional whenever you are in doubt! Never ever take risks on an electrical system
  • Follow the “logic” or convention of starting work on the “dead” end of the system when you do the initial installation of your solar system
  • Install the required minimum lightning protection devices on your system for some additional peace of mind
  • Install at least one in line fuse of the correct “amperage” rating to protect your system and cabling from an over-current incident
  • Make sure that there is an earth leakage unit (ELU) installed in line with the main feed into your system – Also test this unit regularly to make sure that it actually works effectively

That’s it for this week…

Once again, if anything is not clear to you, or if you have any specific questions regarding the discussion this week, please reply to this email and I will gladly assist you to figure it out.  Until next week, when we will talk about the different cable sizes required for your system.

Choosing the right battery technology and size for your needs – (Part 2)

OK, last week we talked about what batteries are, how they work, the different types of batteries and how to care for your batteries.  This week we will focus on doing the calculations to figure out how many batteries of which size you will need for your specific application.

A re-cap of how much energy we need

From a previous blogpost (see the post here: ) we figured out how much energy you will need for your specific application.  I am going to use an example here to explain the mathematics and for us to figure out how to determine what the size of the batteries should be.  So, before you can start to figure out the size of the battery bank, you need to go back to the answers you got when you did the initial calculations for the sizing of your system.  For the example that we will use in these calculations I am going to assume that the batteries are sized for a system that needs to provide approximately 22kW-hours of energy over a 24 hour period of time.  I am also furthermore assuming that most of this energy will be consumed during the day (60%), while the rest of the energy is consumed once the sun has set.  This means that we need to have a battery bank that will be able to give us at least 8kW-hours over a period of about 10 to 12 hours.

This gives us an indication of what the size of the battery bank needs to be to keep everything running from sunset to sunrise the next morning, or for a longer period of time if you want to build this redundancy or autonomy into the system.  There is off course also the options of using grid connected electricity if you are grid tied, or using some other form of energy (like a back-up generator or a wind turbine) to assist with the charging of the batteries.  These additional sources of energy will most definitely lower the size of the battery bank that you will need, provided that you can tap into this energy when you need it.  In this example, to keep things simple, I am assuming that the system has enough solar panels to properly charge the battery bank, and that only a back-up generator is available to assist with charging the battery bank.  [Refer to the blogpost here: Insert link in here… to refresh your memory regarding how to size the solar panels]

How much energy is stored in the battery?

There are again a number of ways in which energy stored in a battery is measured and indicated.  I guess the purest form of this measure is the unit of Joules (or watt-seconds) indicating the energy stored in a battery.  The most common measure of energy storage capacity of a battery is given in amp-hours.  The amp-hour rating of the battery indicates how much charge or energy can be stored in the battery.  A typical deep cycle 12 volt battery will have an indication of Amp-hours stamped on the side of the battery somewhere.  Again, if all else fails, get a hold of the data sheet for the battery from the supplier or manufacturer.  This data sheet will have all the information on it that you will need to do the calculations for the sizing.

There is one other factor that we also have to take into consideration.  The amp-hour rating is always given in terms of a so-called “discharge rate” over a specified number of hours.  [This is the C-rating on the data sheet, eg. C-20 or C-10].  Normally, battery manufacturers give the amp-hour rating (in our example – 235 amp-hours) for a 20 hour discharge period [a C-20 rating].  This means that the energy stored in the battery is approximately 235 amp-hours, provided that the discharge from the system takes place over a 20 hour period.  This can get a little tricky, because the sunset to sunrise part of the day is normally not 20 hours long, but significantly shorter unless you live in a part of the world where you do have very long periods of time during a 24 hour cycle without sunshine.   Here I am thinking of long winter nights and specific changes in seasons.  Make sure that you get the right numbers from the data sheet because some suppliers of specifically cheaper batteries give the C-100 rating as opposed to the C-20 rating.  This makes a significant difference in the calculations and the size of the battery bank that you will need.

In the case of the example I am using, I am looking at a 12 volt deep cycle battery with a capacity of 235 amp-hours, at a discharge rate of 20 hours (C-20).  This information I got from the battery itself (amp-hour rating), as well as from the manufacturer’s data sheet (C-rating).  When we calculated our consumption in a previous blogpost, we calculated this in watt-hours, or kilowatt hours.  We now sit with the challenge of converting amp-hours to watt-hours in order to compare our energy needs with the capacity of the battery.  OK, the calculation is really simple.  In order to “convert” the amp-hour rating to a watt-hour rating, all you need to do is to multiply the battery voltage with the battery capacity in amp-hours.  In the example I am using, I have a 12 volt battery (the battery voltage) and a rating of 235 amp-hours.  This means that the capacity of the battery in watt-hours will be 12 x 235, which is equal to 2 820 watt-hours.  Remember that this is the calculation for one single battery.  In a typical solar system, we normally would connect batteries in series “strings” to make up the system voltage, while we would then connect the battery “strings” in parallel to make sure we have enough “oomph” to provide the necessary energy for our needs.  A picture of a typical battery bank looks like this:


In this picture you can see that we connected 4 batteries in series, to get to the system voltage of 48 volts.  We then connected 4 “strings” of four batteries in parallel to form the battery bank.  The calculation of the total energy that can be stored in this battery bank is done by multiplying the 235 amp-hour capacity per string by four, because we connected four “strings” of 48 volt each in parallel.  So that means that the total storage capacity of this battery bank is 4 x 235 = 940 amp-hours.  Remember that this is a “nameplate” calculation only and you have to use the correct C-rating figures when you do the calculation for your specific application – refer to the paragraphs below for these calculations.

Let’s now do the calculation to get to the watt-hour or kilowatt-hour capacity of this battery bank.  We need to take the system voltage (average discharge voltage) which in this case is 48 volt, and then multiply that by the amp-hour capacity.  This means that we have to multiply 48 by 940 to get a total of 45 120 watt hours.  If we convert this to kilowatt-hours, we get to 45,12 kilowatt-hours.  Unfortunately we can’t use all this stored energy from the batteries (except as promised with the latest battery technology!).  Depth of discharge is the term we use to indicate how much  of the energy stored in a battery bank we will be using with a specific application.

Depth of discharge

Another factor that we have to take into consideration when sizing the battery bank is the so-called depth-of-discharge, or DOD.  This is measured in percentage, and it indicates how much of the energy stored in the battery bank we will be “discharging” from the batteries over the period of time that we need energy to be supplied from the batteries during “no-sun” time.  And yes, this does mean that although we calculated above in the example that there is Most batteries will last reasonably long (close to their expected life, provided you can keep them at a temperature of around20 to 25 degrees celcius) if you only discharge them to about 30% of capacity.

It is prudent to never discharge the battery bank to more than 20% to 30% as a general rule of thumb for everyday use.  If you do discharge the battery bank to more than that on the odd occasion, like when you have no sunshine, then it will not have a huge impact on battery life.  To discharge to a much higher percentage than about 30% regularly will however reduce the life expectancy of your batteries significantly.  I suggest that you work out the capacity of the battery bank so that you regularly discharge to about 30% for daily use, and on the odd occasion to around 70% or 80% over a 24 hour period.

Finally… Sizing the batteries for your application

There are a few more things we need to consider when we calculate the size of the battery bank required for your specific application.  The first one is the level of autonomy that you require.  This just simply gives an indication of how long you want the battery bank to supply energy to your needs before you charge them again.  Autonomy is measured in hours.

If you want a battery bank that will supply enough energy to just get you through the night, you should use a discharge rate in your calculations of battery size of approximately 10 hours (use the C-10 figures for the battery from the data sheet).  If you also want the system to get you through the day (in other words through 24 hours) on the odd day without sunshine, then you need to also check that the battery bank is big enough for this purpose (use the C-24 rating on the data sheet to do this calculation as a check) and that it won’t discharge to an unacceptable level during the 24 hour period.  Remember that the occasional discharge to a little more than the suggested 30% as I discussed earlier is ok if it only happens once in a while.

Another issue that we also have to take into account when we decide on batteries, is the average operating temperature that the batteries will be used at.  In the case of my example that I am using here, the average temperature is taken as 38 degrees celcius.  The higher this temperature is above the rated 25 degrees celcius, the more significant of an impact it will have on the expected life of your battery bank.  In the case of the example, the data sheets indicate that I can expect approximately 82% of the rated life from the battery bank if I use it in general conditions where the average temperature is about 38 degrees celcius.  Off course, our temperature where I live is not always that high, and the average temperature is actually around 29 degrees celcius if you look at the historical figures, but for my calculations I inserted a slightly higher average temperature just to see what it will do to the expected life of the batteries.  You need to decide how you want to look at this specific factor, then decide on the specific battery technology that you will invest in.  Different technologies will have different life expectancies and this information is also found on the data sheet.  If it is not readily available, please contact the manufacturer or supplier of the batteries you are interested in to get this information.  If you live in a very cold part of the world, the same logic applies just in the other direction.  You will probably be in a batter spot if you can heat up the batteries to around 25 degrees celcius.

In our example to calculate the battery bank’s size, we are using 20 kilo Watt hours per day (the 24 hour period), and of this energy requirement, we use approximately 60% during the day and the other 40% during the night.  If the autonomy for this system is really to just get through the night, then the data and the calculation would look as follows:

40% of 20 kilo Watt hours is 8 kilo Watt hours – this is the energy required to be supplied by the battery bank for the period from sunset to sunrise the next morning.  I therefore need to make provision for about 8 kilo Watt hours of energy to be provided by the battery bank over approximately 10 to 12 hours.

We convert the 8 kilo Watt hours of energy required to an amp-hour rating simply by dividing the kilo Watt hours by the nominal battery bank voltage – in my example it is a 48 volt battery bank:  So, 8000/48 = 166.67 amp-hours.  Let’s round it up to 167 amp-hours.  This is what I will need in battery bank capacity to get through the night

If we look at the picture above, which is the battery bank installation, then we know that we have a total battery bank capacity of 940 amp-hours.  Remember however that this 940 amp-hour rating is the so-called “nameplate rating” only.  The figures you have to use for your calculations to get to battery size, will be provided on the data sheet.  You want to get the data for the C-10 rating, as well as the C-24 rating from the data sheet.  Use these numbers for the size of the battery bank in your calculations.  The C-10 rating for the specific battery I am looking at is 190 x 4 = 760 amp-hours.  This is the figure I will use for the calculation of the size of the batteries to get me through the night.  From the same datasheet, the rating for C-24 is 215 x 4 = 860 amp-hours.  This is the figure I will use for the “check” calculation just to make sure that I won’t be abusing the batteries if I discharge them over a 24 hour period without having any sunlight to recharge the battery bank.

If we use the 8 kilo Watt hours during the night, then it implies that the discharge rate will be 167/760 = 21.97%.  This is well within the suggested 20% to 30% discharge rate that we discussed earlier.  I could actually get away with having only three such strings of batteries in the battery bank.  The calculation would then by 167/570 = 29.29%, which is still within the “safe” operating range of discharge for the battery bank. Although it is very close to the top end of the 30% “safe” range.

We also need to check that the battery bank will be able to deal with one day without sunlight, which means that we have to do a “check” calculation using the C-24 rating, and the entire amount of energy required during a 24 hour period, which in the case of the example is 20 kilo Watt hours.  The calculation to get to the amp-hours that we will need is then 20,000/48 = 416.67 amp-hours.  Again, let’s round this up to 417 amp-hours.  This means that we will need a battery bank that can supply 417 amp-hours over a 24 hour period, and still be within its “safe” operating range.  If we calculate the depth of discharge, we know that 417/860 = 48.48% which is a little higher that the 30% depth of discharge that I suggested before.  Remember however that we said that if this happens on occasion only, then it should be OK.  If we only had three strings of batteries in the battery bank, we would only have 645 amp-hours of energy stored, which means that the calculation for the C-24 situation will be 417/645 = 64.65%.  This level of discharge is a little too much for my liking and it is the reason why I would suggest a battery bank of 4 strings of four batteries.

In the end it is really not that difficult to determine the size of the battery bank that you will need to supply in your energy needs.  I hope this clarifies the way in which we calculate the size of the battery bank that we will need for this specific application.  In the simplified summary below, I summarise the process for you:

Simplified summary

A very short summary of how to choose the technology and the appropriate size for the battery bank:

  • Confirm the nominal voltage of the battery bank (actually the system) that you are planning to use [In the above example this is 48 volts]
  • Get the energy usage for your specific installation from the previous calculations you did [In the example used here it is 20 kilo Watt hours of energy used over a 24 hour period, of which 60% is during the day and 40% is during the night]
  • To calculate how many amp-hours of battery storage capacity you will need, divide the night time kilo Watt hours by the nominal voltage [In the example it is 8000/48 = 167 amp-hours]
  • Now, get the C-10 rating for the batteries you are considering from the data sheet. [In the example case it is 190 x 4 = 760 amp-hours]
  • Divide the energy required in amp-hours by the battery bank capacity [In the case of the example above it is 167/760 = 21.97%]  Remember that you want this figure to be within the range of 20% to 30% depth of discharge
  • Do a “check” calculation using the C-24 rating just to make sure that if you do experience one day without sunshine, your battery bank will be able to supply enough energy safely for you to use [In the example case above the C-24 rating was 4 x 215 = 860 amp-hours]
  • You now have to use the full 20 kilo Watt hours that you will need over a period of 24 hours to do this calculation [In our example above it is 417/860 = 48.48%] Remember that we said that if this kind of discharge only happens occasionally, then it should be fine.

So, there you have it as far as battery bank sizing goes!  From next week onwards we will look at cable sizing, protective devices to use, positioning of all the bits and pieces, and putting it all together.  Once again, if anything is not clear to you, or if you have any specific questions regarding the discussion this week, please reply to this email and I will gladly assist you to figure it out.  Until next week!

Choosing the right battery technology and size for your needs – (Part 1)

OK, the next very important step is to decide which battery technology you are going to invest in, and then what size should the battery bank should be to get you through those cloudy days, or in some cases, just through the night!  This aspect of the design of your solar system is so vast that I had to break it up into two weekly posts.  In this first post I am going to work through the basics of how batteries work, and we will then look at the sizing aspect of this exercise next week.  Choosing batteries is most probably the most important decision you have to make regarding your total investment in your solar system.  Make sure that you know what the benefits and issues are with the different types and sizes of batteries before you go out and buy them.  You could save yourself from making costly mistakes if you do this one properly.

A battery is essentially the best known method that we have today to store electrical energy.  That will undoubtedly change in future as new technologies are developed, but for now we have the trusty old lead plates dipped in a sulphuric acid bath, essentially making up your battery.  The lead plates are positively charged while the acid solution is negatively charged, and that is how the energy is stored in the cells of the battery.  The positive lead plate will wear away over time, and that is also the reason why the thickness of the plate is one of the things that determines the lifespan of the battery.

Different manufacturers use different “mixtures” with different characteristics to make up the lead paste that is used to manufacture the positive lead plates.  The specific “mixture” is based on what their research indicates will positively improve the efficiency and longevity of the battery and also includes the purity of the lead that is used in the plates.  Because lead acid batteries are so widely used, lots of research is still being done on improving this technology.  We also expect that other technologies like Lithium-ion, Nickel Cadmium and Nickel Metal Hydride batteries will advance in future and give us “better” batteries – in other words, batteries that can “store” more energy and can give more cycles to prolong the life of the battery.  For now however, we have to settle for the best we can afford in lead acid battery technology.  I can’t however but think of the good old saying that says if we didn’t start thinking differently in terms of transport, we simply would have had faster horses today!  I am therefore hoping that someone somewhere is sitting in the proverbial “garage” tinkering away at the next mind boggling technology that will help us store electrical energy in a much more efficient way that what we have been using for the last 100 years or so!

Most common types of batteries available today                                                                                                    

There are essentially three main types of batteries available today;

Flooded lead acid batteries (FLA) – These are “wet” batteries, the type we all know very well, where you  have to add distilled water to each cell from time to time to keep enough acid solution in the battery for proper functioning.  These batteries do need regular maintenance – apart from “topping up” from time to time you also need to keep the terminals clean by washing it with a water and bicarbonate of soda solution.  They also need to be kept in a ventilated space because they do “breathe” and therefore give off acidic fumes.  From time to time you also need to do an “equalizing charge” on these batteries to help them stay healthy for longer.  This is mostly done automatically by your battery charger, so don’t worry too much about that.  They have the longest track record regarding performance and they also seem to be the most cost efficient of the options we have today in terms of cost per amp-hour.  With these batteries you can also check the charge state with a hygrometer, which you can’t do with the other sealed battery types.

Gelled Electrolyte Sealed Lead Acid (GEL) batteriesThese batteries are maintenance free; no need (or possibility!) to add any distilled water because the acid solution has silica added to it which allows it to form a gel.  Having gel as opposed to the liquid solution in the battery, avoids most of the issues with venting, spillage and off course “topping up”.  The recharge voltage needed for the GEL batteries is normally lower than that of the “wet” type batteries discussed above.  Although the batteries don’t have the normal quantity of acidic gas discharge like the “wet” batteries, they are actually “valve regulated” meaning that they have tiny one way “valves” that allow the gasses that do form, to escape.  The term “vale-regulated lead acid” (VRLA) is also sometimes used to describe these batteries.  These also have a good track record and the fact that you don’t have to do any real maintenance, make them an attractive option for solar installations.

Absorbed Glass Mat (AGM) type batteries – This is the third type of battery that is commonly used in solar installations.  The electrolyte in these batteries is held in a mat of thinly weaved glass strands – enough electrolyte to fulfil the needs of the battery for its rated lifetime.  They are also sealed like the GEL batteries and don’t need any maintenance.  They are also normally “vale-regulated” meaning that they have the same small valves for ventilation as the GEL batteries above.  Although these batteries typically have 1.5 to 2 times the amp-hour capacity for the same physical size “wet” battery, they are about twice the price of the “wet” batteries.  The technical term is that they have a higher “power density”, which is why they are physically smaller, while packing the same amp-hour punch.  When I look at data sheets for batteries, it seems as if the expected lifetime of the AGM type batteries are also significantly higher that the specification for the normal flooded batteries.  The lifetime or life expectancy of a battery is normally given in the total number of cycles that the battery will be able to give.  Remember that all these figures are normally given for batteries operating at somewhere between 20 and 25 degrees celcius, and in near perfect conditions.  Once your batteries are installed and you start using them, many of these variables will be different.  In our case for example, we live in a part of the world where summer temperatures easily and regularly reach the upper 40’s (that’s celcius yes! …up to 48 degrees some really hot days!) and therefore batteries will not last as long as in a controlled environment of 25 degrees celcius.  This is something you have to seriously consider when you decide on the specific battery technology that you are going to invest in for your system.  In the end, apart from the specific conditions under which your batteries will have to work, the budget you have will determine pretty much what kind of battery bank you will be able to buy.

OK, now that we have a fair idea about the different technologies of batteries that we can choose from, let’s now look at how a battery bank is normally charged.

Battery Charging Stages

There are basically three stages in charging a battery bank: bulk, absorption, and float. “Float” is also sometimes referred to as “trickle” or “maintenance” as a charge level.  The three different stages of charge just simply indicate the voltage and current that is used to charge the batteries.  When “bulk” charging, the maximum safe and “allowable” current is sent to the batteries by your charge controller.  You need to make very sure that these limits are set correctly on your system (all three settings, “bulk”, “absorption” & “float” need to be set correctly) for the specific type of battery that you are using.  You can get most of the information that you will need to do the settings correctly from the battery data sheet, or if all else fails, contact the manufacturers to make sure you have the correct values for these settings.  Again, if these limits are not set correctly, you will probably void the warranty on the batteries, and you will probably end up with a useless plastic container full of very expensive lead far earlier than what is necessary.  Setting these limits incorrectly will significantly reduce the life of your batteries.

During “absorption” charging, which is the second stage of charge, your batteries will be somewhere between 80% and 90% charged, which will indicate to the battery charger part of your system that it is time to switch voltage and limit current to the batteries.  Although the charger may still be putting out maximum voltage to the batteries, the current will now be limited.  When the battery bank is essentially fully charged, the system will know that it is time to switch to “Float” charging.  This means that the current and the voltage are further reduced to just keep the charge in the battery at a level where it is very close to the maximum charge it can take.  In this charge state, the amount of gas that is emitted is also lower, and in the end it also prolongs the life of the battery bank.

Batteries and operating temperature

The temperature at which batteries are operating, has a huge impact on the expected life of the batteries.  The typical life expectancy of a battery can easily be halved if you are operating it at about 40 degrees celcius vs. somewhere around 20 to 25 degrees celcius.  The performance of the battery will also be affected by the temperature, although not as significantly as will the life expectancy.  It is not uncommon to see.

So, in essence, if you live in a very cold part of the world, you will need to keep your batteries “warmed up” by using heating pads, while when you live in a very hot climate, you will have to do everything you can to keep them cool.  The ideal operating temperature seems to be around 20 to 25 degrees celcius for most batteries.  Again, it is better to know this when you start using batteries as opposed to finding out about this when you see your batteries deteriorating to an unusable state.  The message is to keep batteries operating at an optimal range of somewhere between 20 and 25 degrees celcius.

OK, we now know how batteries work, which technologies are commonly used in solar systems, and how they are charged.  We also know some important facts regarding the choice of the batteries you will end up using in your system.  We have also seen how important it is to keep batteries at a reasonable temperature when they are in use to prolong their life, or to at least get the rated life expectancy from them.

Here is the simplified summary of this week’s information:

Simplified summary

A very short summary of part 1 of choosing the most appropriate technology for the batteries you will use in your system:

  • A battery is essentially a set of lead plates dipped in a container of sulphuric acid. It stores electrical energy through the difference in charge between the lead plates and the sulphuric acid solution.
  • Three types of batteries are commonly available and used in solar applications. They are FLA (flooded lead acid) batteries, GEL (Gelled Electrolyte Sealed Lead Acid batteries) and AGM (Absorbed Glass Mat) batteries.  Each of these have pro’s and con’s and there are significant price differences among these different types of batteries.  You need to decide on which will be most suitable for you based on the specifics of your application and installation, as well as what your budget will allow.
  • Batteries are charged in a very specific way. There are essentially three charging “stages” – “bulk”, “absorption” and “float” (or “trickle”/”maintenance”) stages.  Each of these stages is designed to address the specifics of the battery in terms of the limits that it should be charged to.  It is important to get these numbers and settings correct to ensure that you get the most out of the life of the batteries that you invest in.
  • Keep the operating temperature of your battery bank somewhere between 20 and 25 degrees celcius – it seems that this is the optimal range for most batteries.

That’s it on the topic of batteries for this week.  Next week we will tackle the next part of the episode on batteries – we will look at the specific sizing of the battery bank that you will need for your specific application.  Until next week!

Choosing the right type and size inverter

We have now completed the sizing of the solar panel array, and the charge controller.  The next step is to decide how “big” the inverter should be.  The purpose of the inverter is to take the DC current from the battery bank (or from the solar panel array) and to “invert” (change) that direct current into alternative current (AC) electricity.  Appliances that we use in our homes all need AC power in order to work, except in the case of specific “DC” equipment that you maybe could have acquired to go camping or touring where you only have access to a DC electricity source (like a car battery).

There are essentially two types of inverters available on the market today.  You will most probably be asked by a supplier if you want a “modified sine wave” or a “pure sine wave” inverter, even before they ask you about the size of the inverter that you need.  To simply explain the difference between these two types of inverters is in the quality of the AC electricity that each of them will produce.  A pure sine wave inverter produces the better quality AC electricity when compared to a modified sine wave inverter.  “Quality” in this sense indicates what the electrical alternating current signal looks like.  Here is a simple sketch of the typical output of a pure and a modified sine wave inverter:


The advantage of a modified sine wave inverter is that it is cheaper than a pure sine wave module, while its application is really restricted to powering up equipment that doesn’t need a good quality AC electricity input.  Most modern items that we use in our homes, like televisions, fridges, microwave ovens and computers, require a high quality AC electricity input to work, and to keep working.  I say “keep working” because these items can get damaged by a modified sine wave electricity source without you really knowing about it until the equipment is damaged beyond repair!  A very unpleasant surprise indeed!  My advice here is to be prudent and invest in a pure sine wave inverter if you are going to use it to power up stuff in your home.  Apart from the obvious advantage of a high quality output, the pure sine wave inverter is also much more efficient than the modified sine wave unit.

There are also “string” inverters, and micro-inverters available to create AC electricity from a DC source, but I will focus on the “centralised inverter” approach in this post.  I will give more details about these other inverters in a future post.

Many modern inverters sold these days are “bi-modal” (can be used in a grid-tied application as well as a stand=alone application), and most inverters have a range of settings that can be changed to customise it for your specific application.  Although this is true, please make sure that the inverter you invest in is appropriate for your specific application, grid-tied, stand-alone, centralised or distributed.

We also get two types of inverter “technologies” available today – one that uses a transformer, and the other that is called a “transformer-less” inverter (TL inverters).  The first one has a huge copper transformer as part of the set-up that is used to “step-up” the voltage, while the TL inverter uses a different technology to do the same thing.  TL inverters are typically cheaper than their transformer based cousins.  My preference and suggestion still is to stick to the proven transformer based inverters that have a god track record.  When you choose the specific brand of inverter that you are going to purchase, please do your homework and find out what others are saying about the performance of the specific unit you want to buy.  I made this mistake when I bought our first 5kVA inverter – it didn’t perform as specified, and after three factory replacements in two months and the factory running out of stock, I had no option but to buy another inverter.  A very, very expensive mistake that you don’t have to make!  Two brands that I have first-hand experience with are Victron and Outback, but there are many more very reliable brands out there – choose wisely, that’s all I am saying.

When “sizing” the inverter for your system, the very first thing you need to make sure of is that the nominal system voltage of the inverter and the battery bank configuration that you are going to set up, are the same.  If you have four 12 volt batteries in series as a battery bank, which makes it a 48 volt battery bank, then you need to make sure that the inverter is also a “48 volt” inverter.  Check the input voltage range of the inverter – they can normally handle quite a range of input voltages (for example from about 36 volt to 66 volt).

The second “sizing” aspect of the inverter that you need to decide on, is the watt or kilowatt rating of the inverter.  This indicates the maximum amount of electrical power that the inverter can continuously produce, measured in kilowatts or kilo volt-ampere.  The relationship between kilowatt and kilo Volt-ampere has to do with power factor in a system and is again one of those things that we don’t have to get into detail about to get to what we want to achieve.  Just know that converting from kilo volt-ampere to kilowatt, we use a factor of approximately 85%.  That means that a 5 kVA inverter will produce approximately 0.85 x 5 = 4.2 kilowatt, in continuous operation.  Another aspect to remember is that the size of the inverter should be approximately the same as the total watts that will be produced by your solar panel array.  This is just a guide or a “rule of thumb” – there is lots of debate about “over-sizing” solar panel arrays, and I also believe that it is wise to do so.  We will discuss that in more detail in a future post. For now, we need to look at the kilo watts that will be needed by all the appliances that you want to power up in your home, in order to determine the kilo watt (or kilo volt-ampere) rating of the inverter.  To get to this number, add up all the continuous ratings of the items that will “always be on”, and then add the watt ratings of those items that you may occasionally use together with all the continuous items.  In my experience, for most four member family homes, a 5kVA inverter will be more than sufficient.  This however doesn’t include electricity for huge power hungry items like air conditioners, water heaters and electric stoves!  If you have any questions about saving before you start to design your system, please have a look at the earlier posts in the archive section, where I discussed these issues in detail.

These are the two most important measures or “sizing issues” that you need to be aware of when purchasing an inverter. Then there are the other obvious things like the dimensions of the inverter and the temperature range over which it will operate effectively, that you also need to make sure will work in your specific application.  Where we live, temperatures in summer can go up to around 48 degrees celcius, which means that the inverter must be able to still operate under these conditions.  The specific inverter we eventually settled on, has a temperature range of operation of minus 20 degrees celcius to +60 degrees celcius, which is quite sufficient for our application.

There is another aspect that you also need to look out for.  It is called the “peak power” of the inverter.  This is normally substantially higher than the continuous power rating of the unit, and it takes care of the so called “surge currents” that are drawn from the inverter by for example anything with a motor or compressor in it, when they start up.  When a fridge or freezer “starts up”, it draws quite a bit more current than when it has “settled”… This is what is called the “surge current”.  The same is true for an electric motor when it starts up.

In the case of the inverter we bought, the “peak power” rating is 10 kilowatt.  This is more than enough for anything that we will ever power up from the system.  Remember that this “peak power” can usually only be delivered for a few seconds, but it is enough (in most cases) to get things started up.  If you have particularly big loads or equipment that needs to start up, there are other ways as well that you can do to get a “softer” start.

A last thing to mention is the efficiency rating of the inverter.  It is normally called “maximum efficiency” and it is given as a percentage.  Again, there is lots of debate about the dependability of this figure as stated by most manufacturers, the argument being that the figure should be lower!  Most inverters these days have efficiencies in the upper nineties, which is fine for a home application.

Simplified summary

A very short summary of how to choose the correct type and size for the inverter:

  • Make sure the nominal voltage of the battery bank and the inverter is the same (or the voltage range of the inverter can handle the battery bank voltage)
  • Add up the watt ratings of the continuous loads in your home, add the other items that you will occasionally use (like a microwave oven, a washing machine, or a hair dryer), then make sure that the inverter’s kilo watt (or kilo volt-ampere) rating is higher than this peak demand kilo watt number for your application
  • Check that the “peak power” rating of the inverter will be sufficient for your application
  • Make sure that the dimensions and the inverter and the temperature range over it will operate effectively is sufficient for where you live
  • Whatever you decide to buy, please make sure that you buy a reputable brand – talk to other users and make sure that claims made by the manufacturer in terms of performance and service, are actually proven and real – it will be an expensive mistake if you only realise afterwards that the brand is not dependable!

That’s it for this week.  Next week we will look at batteries and how to balance all the components to work together in “symphony”.  Until next week!


Choose and size your charge controller

We have completed the first two steps of sizing a solar system that will provide in all your electricity needs.  We now know how much energy (measured in watt hours or rather kilowatt hours for most of us!) your system will have to provide to power up all those things that you can’t go without!  We also calculated, in the last post, how much sun exposure you get where you live, and then calculated how many solar panels you will need to install to make sure you collect enough energy from the sun during the day to provide enough energy to your system.

The next step is to decide on which type of technology you want to use for your charge controller, and what size it needs to be.

Charge controller basics

A charge controller is a specific kind of voltage regulator and is a necessary part of any modern power system, whether you use solar, wind, hydro, a generator, the electricity grid, or any other source of energy to charge batteries.  The reason for this is that the voltage that you receive from your solar panel array will fluctuate according to the sun intensity.  We therefore need a piece of equipment that is installed in between the solar panels and the batteries that will regulate this voltage so that it is at an optimal level most of the time.  The purpose of the charge controller is obviously to “regulate” a fluctuating voltage that goes to the battery banks, but also to prevent reverse current flowing from the batteries back to the solar panels at night (“reverse voltage” – see explanation further on).  It also needs to protect against battery overcharge and under-voltage or overload conditions.  Some of these charge controllers also have fancy LCD displays, while others have basic LED or metered displays of voltage and sometimes also of current.  The bottom line is that you need one of these to make sure you have the best efficiency from the solar panels transferring energy to your battery bank, and that you have some peace of mind knowing that some protective measures are in place to protect your investment.

Protecting the batteries from overvoltage and under-voltage situations is particularly important to prolong the life of the batteries.  Once you abuse a battery, you will take many cycles off of its potential life, and that is not something you want to do!  Batteries are an expensive part of the system and you want to get as much out of them as possible, so in general we want to look after these batteries as best we can!

Types of charge controllers or regulators available in the market

There are different types of regulators or charge controllers available on the market, and it will be good if you knew what these are all about before you decide to buy a specific control unit.

The “oldest technology” that we still see in use today is a simple “on/off” type regulator.  These regulators normally use relays (or electronic switching) to simply switch the voltage to the batteries on or off, and that way they regulate the flow of energy to the batteries.  These also regulate the voltage to the battery bank, and some of them have the ability to disconnect when overload or under-voltage conditions take place.  They would normally have a function that allows you to set the different voltages – under-voltage, overvoltage, etc.  Some will also have a function to prevent “reverse voltage”, which is a condition that happens after sunset in a solar system where the voltage on the solar system array goes lower than the battery voltage.  This function simply protects against energy flowing back from the battery bank to the solar panels, really resulting in an energy loss.  These basic “on/off” type charge controllers are still in use today because they are pretty dependable and last.  They are however less efficient than any of the others available.

Another type of charge controller that is widely used is called a Pulse Width Modulated (PWM) charge controller.  Again, don’t worry too much about exactly how this technology works, we don’t need to go into that for our purposes here.  This was an “upgrade” in technology from the rudimentary “on/off” type controller.  A PWM controller is softer on batteries because they reduce current gradually and work by matching the solar panel voltage to the required battery voltage.  This means that the excess energy is normally simply dissipated through a heat sink, but it also means that we are not using all of the energy available to us from the solar panels.  This is also the reason why the PWM type controllers are about 20% to 30% less efficient than a MPPT type controller.  If you do choose this type of controller (or you have one that you want to use), you need to make sure that it has set points for the different voltages that you would want to set, or make sure that the standard settings on the unit is correct for the specific battery bank that you are going to use.  This limits you in terms of flexibility of your system later on.  I would therefore recommend that you consider rather investing in a MPPT type controller if you have the option of buying from scratch.

The regulator that is preferred and used most often in solar installations these days, is called a Maximum Power Point Tracker (MPPT) charge controller.  Apart from doing all of the things I mentioned above regarding battery protection and so on, this slightly more intelligent piece of equipment will also make sure that the maximum amount of energy is transferred from the solar panels to the batteries.  They are the most efficient controllers available today and can reach efficiencies in the upper nineties, 94% to 96%.  An MPPT controller continuously monitors the energy that is available from the solar panels, and adjusts the voltage output to the battery bank so that the maximum amount of energy is transferred to it.  Although the MPPT type of charge controller is a little bit more expensive than the more general regulators that only protects your battery and regulates the voltage, it is well worth your while to invest in this.  The increased efficiency that you get from it makes up for any price difference in a very short while!  Consider getting this kind of charge controller for your system.

OK, that is enough said about what types of charge controllers are available, and which one is the best for you to choose.  Let’s now look at how to figure out how big this piece of equipment needs to be for your specific application.

Sizing the charge controller that you will need

The “size” of the charge controller is determined by how much energy you want to transfer from the solar panel array to the battery bank.  The “size” of a charge controller is normally given in amps [A], which indicates the maximum amount of electrical current the charge controller will be able to handle.  This simply means that this rating in amps (remember that this is the measurement unit for electrical current) is the maximum that the charge controller will be able to manage or accept from the solar array.  Good practice is to always make sure that you have some “spare capacity” when you size the controller – Apart from just feeling a little more comfortable knowing that there is some spare capacity and that the controller will not be working at full capacity all the time, it is comforting to know that if I would like to add some more panels to my solar array in the future, there is the possibility for me to do that without having to buy another charge controller.  How much spare capacity is needed?  Most people will suggest that you work on a factor of about 25%, which means that you need to work this factor into your calculations when determining the size of the controller.

The very first thing you need to determine, even before looking at the Amp rating of the controller, is to just make sure that the charge controller is appropriate for the battery bank voltage that you are going to use.  Here, you basically have three options – you can configure your battery bank as a 12 volt, 24 volt or 48 volt setup.  This just simply determines how you will connect the batteries in series to get to the specific system voltage of 12V, 24V or 48 Volt.  We will still talk about this in more detail in a future post, but for now, just know that the lower the battery bank voltage is, the thicker the cables will be that you need to safely connect the whole works together.  That was the main reason why we decided to configure our batteries as a 48 volt system.  We therefore needed significantly smaller gauge cables to do this and it has been working very well for us from the first day!  Remember that the downside of choosing 48V over for example 12V is that you need a series of 4 batteries connected together to get to 48 volts.  It implies that you will need more batteries, but if you are going to need the stored energy anyways, this is not really such a big deal.  So, the bottom line is, choosing 48 volts as your battery bank configuration has some advantages over the other two possible configurations that you could use.

Now that you have made sure that the controller will be able to deal with your battery bank voltage, you need to make sure that the size in terms of the current that it can handle will be enough.  MPPT charge controllers come in sizes ranging from about 20 Amps to over 100 Amps.  Typical sizes are 20 amps, 40 amps, 60 amps and 100 amps.  Let’s now have a quick look at how you figure out how many amps you can expect to get from your solar panels.  You will need some technical information about the panels to do this, so get a hold of the data sheet or “spec sheet” as it is also referred to, for the specific panels that you are planning to buy.  Here is an example:


I am using the 250 watt panel from this sheet as an example, as this is exactly what I have installed in my system.  From this sheet, you can see that the “Maximum Power Current” (Imp) for the 250w panel, when one of these panels is generating maximum energy, will be 8.31 amps.  [This is referred to as the “Maximum Power Current”, denoted by the symbol Imp – this is the data you are interested in for this calculation].  I have an array where I connected 3 panels in series, to increase the voltage I get, and I then have four of these “strings” in parallel.  This simply means that I have to multiply the maximum power current that I can expect by 4, which is the number of “strings” that I have connected in parallel, to get to the maximum current that I can expect from the solar panel array.  The maximum current that I will therefore expect from this array will be 4 x 8.31 = 33.24 amps.  Now, remember that we spoke about a “safety factor” or a bit of “spare capacity” that we want to include in the design.  We suggested a 25% “safety factor previously, and that means that we have to take this into account in this calculation.  We therefore multiply the maximum current that we expect to get by a factor of 1.25.  This means that I need to work with 33.24 x 1.25 = 41.55 amps.  If I have to choose the size of the charge controller that I will use, I have to make sure that it will be able to deal with a minimum of about 42 amps.

I also mentioned that I may want to increase the solar array that I have currently installed in future, which means that I have to make sure that the size of the charge controller that I choose will have some additional spare capacity for me to do this in future.  My overall system size will allow me to have an array of about 5,000 watts [or 5 kilowatts] of solar panels installed.  If I simply continue with the parallel strings I currently have as a solar array, and add two more such strings, then I have to make sure that the charge controller will be able to handle six such strings put together.  The math then is 6 x 8.31 = 49.86 amps, and if I add the safety factor to this: 49.86 x 1.25 = 62.33 amps, it means that I will need to choose a charge controller that can deal with at least 63 amps.  OK, a lot said, but that means that I will consider the next bigger “size” of charge controller bigger that the previously mentioned standard of 60 amps.  I chose to get a 100 amp charge controller which will give me more than enough capacity for my needs right now, and it will also allow me to significantly increase the size of my solar panel array in future.

I also took into account that we live in a very hot part of the world, and I know that electronics don’t like to be too hot when they work, which is another reason why I went for the significantly bigger charge controller.

This is a picture of what my specific installation looks like (It’s my own hand drawn sketch!).  You can see how three of the panels are connected in series, while I then have four of these “strings” that I connected in parallel.


OK, we have now determined two things in terms of the charge controller.  Firstly we made sure that the controller will be able to deal with the specific battery voltage configuration (12v, 24v or 48v), and we also made sure that the current that it can deal with will be enough for our specific application.

There is a third aspect of the charge controller that you need to be aware of and that also needs to be taken into account.  That is the maximum voltage that the charge controller will be able to handle from the solar panel array.

Here is a picture of the “spec sheet” or data sheet for the charge controller I bought.


From this data, you can see that the maximum voltage that this controller will be able to handle is 150 volts.  This means that I need to make sure that the way I “string” together the solar panels in the array, has to generate a voltage that is lower than the maximum of 150 volts.  Let’s now again look at the data sheet for the solar panel.  From this sheet, you can see that the “Open circuit voltage” for the 250 watt panel is given as 37.4 volts.  Remember that I put three of these panels in series, which means that I need to multiply this voltage by 3 to get to the maximum voltage that the array will produce.  The math is 3 x 37.4 v = 112.2 volts.  This is within the specification for the charge controller, which means that it will be good for my specific application.  I first had the panels configured as 4 panels in series, which implies that the maximum voltage I would then get will be 4 x 37.4v = 149.2 volts.  You can clearly see that I will then sit with a situation where the voltage generated by the solar array is just a little too close to the maximum that the charge controller can deal with.  I did this initially and could figure out why the charge controller wasn’t working well!  When I then reconfigured the solar panels to have only three of them in series, which meant that the voltage from the array went down to 112.2 volts as indicated above, the charge controller worked like a charm!  I hope this example helps you not to make the same mistake I made when I first set the whole thing up!

OK, that’s really all you need to choose in terms of the size of the charge controller.  Below is a simplified summary like I always add at the end of the post.

Simplified summary

A very short summary of how to choose the appropriate size for your charge controller is:

  • You will need to get the data sheets or “spec sheets” for the solar panels and the charge controller to get the data that you need for the calculations
  • Find the data on the solar panel data sheet for Imp [Maximum power current for the solar panel], and Voc [Open circuit voltage]
  • Find the data from the charge controller data sheet for system input voltage, and current rating
  • First of all – choose a charge controller that is appropriate for the battery bank voltage that you will end up with [My example above – I connected the batteries in a 48 volt configuration, the charge controller needs to be able to handle this]
  • Choose a charge controller with an input voltage high enough for the specific configuration of solar panels in the array that you are going to install [My example above – 150 volts maximum input to charge controller, my array gives a maximum of 112.2 volts]
  • Calculate the maximum current that you will get from the solar array – choose a charge controller that will be able to deal with this current [Remember to allow some room for future expansion, and remember to multiply by 1.25 for the “safety factor”]. My example above – solar array will give maximum of 63 amps in the future – I chose a 100 amp charge controller
  • These are the three factors that you need to decide on when choosing a charge controller

This week’s step in sizing the solar system that you will need was rather simple, but it is really important none the less.  Next week we will get into the whole space of choosing an inverter and then making sure that the size of the inverter is big enough to generate the amount of electricity that you will need.  Until next week, happy calculating!

As always, if you have any specific questions that I can help with, please respond to the email, or simply add a comment below the post.

How much sun exposure do you get?

Now that we have figured out how “energy hungry” the stuff is that you use on a daily basis, we need to find out how much sun exposure you get where you live (other terms used for sun exposure include “solar radiation”, “insolation” and “irradiance”.  They all mean pretty much the same thing, for our purposes anyways).  We will use this information directly when we calculate how many solar panels you will need to collect enough energy from the sun to generate sufficient electricity to power up all your stuff.

Methods and measurement of sun energy

There are a number of ways in which weather stations and other sources of this kind of data will tell you how much sun energy you receive on average where you live.  One method is to indicate to you how many Mega Joules of solar radiation per square meter is received on a flat surface at a specific position on the earth – typically given as a yearly number, and it is normally referenced to a flat surface, meaning that no tracking of the sun, or any elevation of solar panels towards the sun is included in the results [Measured in Mega Joule (MJ) per annum, per square meter].  Let’s not even go there!  This kind of figure is interesting but it is not what we need – way too technical for our purposes!

Another way in which solar radiation, or the amount of solar energy that is received on average at a specific place can be indicated, is in kilowatt hours per square meter.  This is yet another interesting number, but again not exactly what we are looking for.  We need information that simply indicates to us that at the place where you live, you receive a specific number of sunlight hours per day.

Together with the number of sunlight hours per day, there is another interesting bit of very important and useful information that is normally given, which is the average “sunny” daylight hours – normally given as a percentage.  You will either get a percentage of sunlight hours, or a percentage for the cloudy or murky parts of the day. Again, these are all average numbers.  This number gives us an indication of how many of those average sunlight hours per day we can bank on for proper collection of energy from the sun.  We use this number to make sure that we compensate for the start and end of the day by designing the panels slightly bigger than just what the average daylight sun hours would have required.  This will take care of the fact that on a typical day, during the first few hours (1 to 1 and a half hours) of the day, the sun is pretty low on the horizon, and therefore the energy received by solar panels will be significantly less during the first few hours of the day.  Exactly the same goes for the latter part of the day, when the sun starts to set.  During the afternoon, the energy intensity is again lower, implying that we have to compensate for this by over-designing the size of the solar panel array.  This over designing of the size of the solar panel array will also take care of the (hopefully!) small percentage of cloudy or overcast days that you will experience.

Finding the data

To find this information, please use google and type in the search term “sunlight hours per day in [City or place where you live]”, or “sun exposure in [City or place where you live]”. You will find a number of sites that will give you data – look for a site that gives you the numbers I discussed above.  We need the average number of hours of sunlight per day, as well as some indication of what percentage of this number is “usable” sunlight.  We now have the relevant information to work out how big the solar array needs to be to collect enough energy from the sun for the place where you live.

An example – the real figures

Here is an example of a data set that indicates the average number of sunlight hours received at a specific place (or town) for each month of the year.  It also indicates the average percentage of cloudy (overcast) days per year.  The example given here is for Johannesburg, in South Africa. This is what it looks like:


From the above table we can get the average daily number of sun hours for Johannesburg, as well as the percentage of cloudy days, or “unusable” sunlight hours per year.  This is the information that we need to figure out what the size of the solar panel array will need to be.

Here is the math for how we use this solar radiation information to calculate the size of the solar panels that we will need:

From the above table we can see that the annual average sunlight hours per day, is given as 08:42, meaning that the average sunlight hours per day is 8 hours and 42 minutes.  Let’s say that the average sunlight hours is 8,5 hours per day, just for practical purposes.  From the table we can also see that the average “usable” sunlight hours per day, is on average 72.6%.  This means that the number of “usable” sunlight hours per day is 0.762 x 8.5 which gives us an average number of 6.47 hours per day, or 6 hours and 28 minutes.  To me that looks close enough to 6.5 hours of usable sunlight hours per day.  These are the numbers that we need to size the solar panel array.

To calculate the size of the solar array, you will also need to get the average number of watt hours per day that you plan to use, from the previous blog post and the usage calculator (or from your utility bill).  Let’s take an average of 25,000 watt hours per day for our example for Johannesburg.  The average number of “usable” sunlight hours per day was 6.5 hours.  Take the required watt hours per day (25,000 watt hours) and divide that by the number of sunlight hours per day: 25,000/6.5 = 3,846 watts.  This is the number of watts that your solar array needs to generate per hour of “usable” sunlight hours in the day.  For practical purposes, and to make sure that we have yet another little bit of leeway included in our calculations, we need to find solar panels of approximately 4,000 watts of capacity.  (It is only really about a 4% over size of the solar panel array!)

Sizes and the number panels we will need

Solar panels come in many different sizes and now we have to decide what will be an appropriate size of panel for us to use for this application.  A pretty standard size is 250 watts per panel.  This number may vary slightly depending on which supplier you use, anywhere between 245 watts and 265 watts per panel.  If we use a size of 250 watts per panel for our example, we simply have to divide the total number of watts per hour that we need by 250, to get to the number of solar panels that will be needed in our solar array.  This means 4,000/250 = 16.  This means that if we use solar panels of 250 watts each, we will need 16 of these panels to supply in our needs.  I will discuss the way we wire these panels, and integrate them into our system in a future post, but for now, we know how many solar panels we will need to collect enough solar energy from the sun to deliver enough electricity for our use.  This is what we wanted to achieve with today’s discussion!  See, it really isn’t that complicated at all!

When I did the math for the place where my wife and I live, the results were pretty much the same.  We use slightly less kilowatt hours per day (approximately 21 kilowatt hours) and our sunlight hours is slightly more than for Johannesburg (8.7 hours per day, with 23% low intensity sunlight), which means that we only needed to install 12 solar panels in our solar array.  We live in the far north western part of South Africa, close to a town called Lephalale, in the heart of the so called “bushveld” area.  Farming and mining are the main economic activities here.  We are about 80 km away from the border with Botswana on the North West.  We are absolutely blessed with sunlight, and it was a no brainer for us to decide that we will be using solar to generate electricity for our needs.

We don’t experience a lot of wind which means that wind energy generation is not really feasible for us.  We do have some wind, but it is concentrated during specific months of the year, and it is not sustained.  If you happen to live closer to the coast, or in a place where wind is prevalent, then you should most definitely consider using wind energy generation together with your solar power system.  I will discuss the integration of different technologies of power generation into one system in a future post.  There is again a whole bag of tricks that you need to know about when it comes to wind energy!  But that is for later…

If the difference between summer and winter sun exposure where you live is significantly different (less in winter), you will essentially need many more panels for winter time to produce the same amount of energy.  In some cases this may imply that you will need almost twice the number of panels for supplying your electricity in winter.  Here you need to make a call to see if you can reduce your load during winter (pretty unlikely!), install more panels, or install a generator to use as backup energy supply for the winter months.  The costs, both capital and running costs, will be what will help you decide in this case.  Again, if you have any specific questions regarding these decisions, please drop me an email or comment below the blog, I will gladly assist!

We decided in our specific situation that 12 solar panels will be enough to power up everything we need for probably about 75% of the year.  We then use a backup generator to help a little when we really have long periods of cloud cover, rain or reduced sun intensity.  It has worked fine for us, and the cost elements have been manageable.  My thinking is however to install more solar panels as our budget allows, to avoid using fossil fuels at all to power up all our needs.

Simplified summary

In short, to determine the number of solar panels that you will need in your solar array:

  • Get the data that indicates the average number of sunlight hours per day for the area where you live (Do a google search)
  • Get the percentage of “usable” sunlight hours per day from a similar source (probably the same data source)
  • Do the sums to get to the average number of “usable” sunlight hours per day for the area where you live
  • Use the number that you calculated from the previous blogpost as the total number of watt hours per day that you will require for all your electricity needs (in watt hours per day)
  • Divide the number of watt hours needed per day by the number of “usable” sunlight hours per day – This indicates the number of watts that your solar array has to produce per hour of “usable” sunlight per day
  • Take this number and divide it by the size (power rating) of the solar panels. This will give you the number of solar panels that you need to install in your solar array

I hope you enjoyed reading the post this week.  Remember that we are now one step closer to knowing what the size of our total system will have to be.  In a future post I will talk about tilt angles and tracking systems for solar panels, when we get to the physical installation of the system. Remember however that I strongly urge you to only do what you can safely do yourself, and to please enlist the services of a professional installer or electrician in your area to help you with the connections, commissioning and starting of your system.  In most places in the world, this is a legal requirement as well – please be safe and do everything you do within the confines off the law!  Remember that the idea with these discussions is to educate you and to help you get better informed in terms of making the right choices regarding your off grid and solar system, not to get you to necessarily install everything yourself.  There is huge value in just knowing what you are talking about when you have to negotiate with a supplier in your area when you purchase your system.  Happy calculating and planning until next week!

Some more thoughts on figuring out how much electricity you use, and a few more tips on reducing your electricity usage…

Last week I explained how to figure out how much energy you use, and therefore, how much electricity you will need to generate if you decided to go completely off the grid.  Just as an interlude before we get to calculating your electricity usage by using your utility bill, I want to mention two more tips regarding reducing your electricity usage without much effort or cost.

More simple tips for saving electricity – replacing lights and lowering the setting on your hot water system’s thermostat

Two more tips that I would like to share with you that will help in your quest to reduce your energy usage before you figure out the size of the system, is to consider replacing all the lights in your home with either energy saver lights, or better still, LED lights.  If you replace a normal 100 watt incandescent light bulb with an energy saver light, you will be able to generate the same amount of light with approximately 10w to 15w of power.  If you decide to go all the way and replace the incandescent light bulb with a LED light, you will probably be able to generate the same amount of lights with approximately 3 to 6 watts of power. It is very clear that by simply replacing the technology that you use in your home to generate light, you can save a lot of electricity.  Now, these new technology lights are more expensive than the original incandescent light bulbs that we all used way back when, and it is therefore normally a process of progressively replacing lights as they stop working, or working out some form of a program for yourself to replace all these lights over a period of time.

We have replaced every single light in our home with LED lighting, and we did this over a period of a few months.  I first replaced all the lights that we use regularly, basically on a daily basis.  We then replaced those lights that we don’t use every day, and finally we replaced all the remaining lights that we only use every now and then.  We now use no more than approximately 200 watts of power to generate all the lighting that we need, if we would switch on all the lights at the same time.  Included in this is off course all the lights we have installed, and we don’t ever use all of them all the time.  We therefore don’t use 200 watt hours every hour after sunset, we only use the watt hours from those lights that we actually use.  We use only about 100 watt hours for all our lighting.  I know this is obvious for some, but I had to just explain this in very simple terms.  Although lights seem like small energy consumers when compared to other bigger items in the home, in the long run, it makes a significant difference when you replace all the lights in your home with LED technology.  When we replaced our lights, we did our homework and shopped around and found suppliers in our area that had really reasonable prices.  In the end it didn’t cost us a lot to replace all the lights.

There is the argument that says that LED and energy saver lights don’t generate as much light (measured in lumens) as an incandescent light bulb does.  This may be true, but when you compare the amount of energy consumed by incandescent technology, it is a no brainer to replace all lights with a more efficient technology.  In some places the sale of incandescent lightbulbs is now illegal or heavily taxed, forcing all of us to convert to either energy saver lights, or LED technology.  A good incentive I guess to get all of us to at least consider going a little greener regarding lighting.

The second point I wanted to make regarding reducing your electricity consumption before sizing the system that you will need is about hot water systems.  Again, replacing a traditional electric water heating system can be a costly exercise, and most of us will probably have to budget for this as a rather large capital expense over time.  The one simple thing that doesn’t cost a dime though, is to turn down the thermostat on your current electric water heating system, from the usual 85 or 90 degrees celcius, to around 50 to 55 degrees celcius.  By simply doing this, you will save a huge amount of electricity, and you will very quickly realise that water at around 50 degrees celcius, is more than hot enough to do anything in and around the home.  Another thing you can consider doing if you are diligent and will remember, is to only switch on your electric heating system for about 2 hours per day instead of leaving it on all the time.  We found when we were still living in the city, that by doing this we also saved a huge amount of energy.  We did in the beginning have one or two mornings when we didn’t have hot water (I forgot to switch it on, and I paid dearly for it by having a cold shower!) but we got used to the rhythm very quickly and in the end, the saving was well worth it.  So there you have it, some more things to consider to reduce your overall electricity usage.  Let’s now get back to calculating the size of your system.

Getting back to calculating your electricity usage…

Another method for getting an idea of how much energy you consume, and off course given that you are currently connected to an electricity utility supply network and you receive a bill each month, is to simply look at the bill you receive.  To get to an even better estimate, have a look at bills from the last 6 to 12 months, and calculate the average figures – this will indicate to you how much electricity you consume on average per month.  [The numbers you are interested in from the electricity bill are those indicating kilowatt hours of consumption per month].

Now remember that the reason why I gave you a different approach last week is that when you look at your current electricity bill, what you see is your current consumption.  In a previous post I spoke about how to reduce the amount of electricity you will need when you decide to go off the grid.  A few of the things that were mentioned included among others cooking with gas instead of electricity, and making use of the sun to heat up water for use in your home.  You really don’t want to just simply take the numbers from your current electricity bill and then decide based on that how big the system needs to be to get you off the grid.  If you do this, you will probably end up figuring out that you will need a system that will be very expensive, and one of the things that I would imagine all of us want to get right on this journey, is to see if we can’t live comfortably, while using less electricity.  Just one item, the hot water heating system, will very easily add double the amount of electricity to your daily usage figures, and if we don’t at least consider taking this one item off the electrical grid, we will need a very large system to power everything up.

In a future post I will show you what we did in our home to make sure we always have hot water by simply using the sun, and then installing a gas powered back-up system for those cloudy days.  There really is no need to use an electric hot water system anymore these days – there are really great options to consider heating water with different sources of energy.

Using your electricity utility bill to calculate your usage

When using your electricity bill to get an idea of how big the system will have to be to provide for all your creature comforts, you can “reverse engineer” the final figures from your electricity bill.  You need to take the average number of kilowatt hours that you consume per month [The figure you calculated above as an average over 6 or 12 months], divide that number by the number of days in the month [Use 30 days for practical purposes], and presto – you have the average usage figure per day, in kilowatt hours.  From this figure you can now do the “reverse engineering”… subtract the number of kilowatt hours of energy used by those items that you replace with for example gas or sunlight, [also consider subtracting those items you will remove simply to save on electricity usage] to get to the final number of daily kilowatt hours that you will need to maintain your current lifestyle.  [A tip here – make sure that you work with daily kilowatt hour numbers for all calculations… Else your calculations are going to get confusing or amusing very quickly when you mix numbers for days and months… – just a caution, again this will be obvious for some people out there!  IF you have any specific questions regarding the numbers, please email me, I will gladly assist!]

Here is an example: 

Let’s say that you calculated the average number of kilowatt hours used in your home per day to be 35kwh per day.  If you then decide to replace your hot water heating system with a solar system, and you decide to ditch the electric stove and over for a gas stove and oven, you could be saving an average of 8 to 10 kwh per day, if not more.  In this example, use a “savings figure” of 10kwh per day, which means that the total energy you will need in a day will be 25kwh. That implies that the system you will need to keep going the way you are, will need to supply much less electricity and deliver much less stored energy during night time as well.  We will use this number in a future post to determine the size of the battery bank that you will need to power up everything from sunset to sunrise.

Whichever method you decide to use to determine how much energy you will need – make the effort and figure out how much energy you will need.  We can then work from there to determine the size of the system that you will need to power your home with solar energy.  In a future post we will still get to the other number that will be important for you to decide on the size of the system you will need – the maximum demand number; which is the maximum number of kilowatts [Note: not kilowatt hours this time!] that you will typically use from the system.  This number is given in kilowatts (or kilo volt-ampere (kVA)… but as we said before, use kilowatts for ease of the calculations and to keep things simple!).  We will get to this one in a week or two – let’s work this step by step!

Simplified summary:

So there you have it – Three more aspects,

  • First, you can use your electricity utility bill, average the numbers over a period of time (6 to 12 months, or really any number of months that you want…) and then “reverse engineer” the figures by subtracting your energy savings to get to the final number of kilowatt hours of energy that you will need to maintain your lifestyle.
  • Secondly, consider replacing the lights in your home, over a period of time, with either energy saver lights, or better yet, LED technology. This will have a significant impact on the total amount of energy that your system will have to generate.
  • Thirdly, if all else fails and you have to budget to do the replacement of these items over time – one thing that you can do immediately is to simply turn down the thermostat on your current electric heating system from the usual 85 or 90 degrees celcius, to approximately 50 to 55 degress celcius. Nobody will even notice that you did it – water at around 50 to 55 degrees celcius is more than hot enough to do whatever it is that needs doing in the home!  You can also decide to switch on the electric water heating system for about two hours per day only in order to also save a whole lot of electricity.

Happy saving, until next week, when I will show you how to figure out how much sun energy you can bank on to collect through your solar panels!

Figuring out your usage so that you can size your system…

Once you have made the decision that you want to get off the grid, the most important consideration is to determine how much energy (electricity) your system will need to supply in order for you to power up all your necessities and comforts.  First, I will address the option of going off the grid completely.  In a following post I will help those who want to get off the grid systematically, by doing it bit by bit.  You can however follow along, as you will have to do the same preparations to determine the size of the system you will need.

You can also buy your own data logger but for the limited use that we will have for this kind of equipment, this is not advised.  You can purchase a small inline data logger that you plug into the wall socket that feeds a specific item and determine the usage in real time, but again, I am not sure this is worth your while spending money on.  Rather keep your money and invest it later on in a proper true RMS multimeter or some other equipment that you may need and that you will use much more often.

The data logger route

The best way to determine this would be to contact a service provider in your area who has a data logger (a rather expensive piece of equipment!) and then to make a deal to have it installed at your home for a period of anything from a week to a month.  They normally charge between $10 and $20 per day to have the data logger installed at your place.  The data logger will measure your energy consumption (both peak energy demand [kilowatt] as well as total energy used [kilowatt hours]) on a timeline over the period of time that it is installed for.  The service provider will usually give you a report with some graphs and tables indicating to you what your peak demand is, as well as what your average consumption over the period of time is.  You can then use this information to determine the size of the system that you will need.

The manual route (DIY option)

There is also the manual way of doing it.  This is probably the preferred method for most of us to get to understand how much energy we will need.  It boils down to you making a list of all the things in your home that use electricity, and then figuring out how much energy each item uses, and for how long during the day or night you actually use them.  Here is an excel spreadsheet that you can use to do an inventory of the items in your home that use electricity.  I have not protected any parts of the spreadsheet, which allows you to manipulate it the way it suits you.  Try not to mess with the formulas in the “totals” columns, just add the data that you collected into the appropriate cells.  If you have more items that what is on the list, simply insert more lines into the worksheet and then add the details for the additional items.  You can also simply remove or replace items in the spreadsheet list that you don’t have in your home.  Remember to also copy the formula from the line above into the new “totals” cells you created if you add any additional lines to the spreadsheet.  If you do mess up the spreadsheet for some or another reason, just download a new version from the website and start over, or make sure that you save an unused copy on your computer so that you can start over if anything gets corrupted!  No worries, play around with it and customize it so that it reflects your specific situation.

(You can download the spreadsheet by clicking on the link below!)


I also indicated typical wattage ratings for electrical items that we all use on a regular basis in the spreadsheet right next to every item on the list.  You will undoubtedly have some items that nobody else has, and that is why I am giving you a spreadsheet that you can manipulate to suit your specific needs.  It is a slightly different approach from what you will normally find on the internet, in the sense that most “calculators” you will find out there have protected cells or they  are masked in some other way and they don’t show you what the formulas are that are used to calculate your usage.  I will share all the formulas with you (don’t worry, it really isn’t rocket science!) so that you also get to know exactly what you are doing and where all the final numbers come from.  This also helps in seeing where the big power users are and where you need to focus if you want to get your usage down before you decide on the size of the system that you will need.

You essentially need to walk through your home while you make your own list of items on a piece of paper, including the power that each consumes.  Walk into the first room in your home and start making your list of lights, items that are plugged into wall sockets, and items that are wired directly into your electricity network.  To figure out how much power each light uses, look for the “watt rating” printed on the glass/plastic cover or the stem of the light bulb.  The convention is for manufacturers to print the “watt rating” on the glass/plastic cover of each light bulb.  This one is rather easy to find.  Add items to your list as you move from room to room.  Also check what you have outside the house, and also note those items that may be installed in the garage or other outbuildings.  Do the inventory as thoroughly as you can – it will help a lot with the process of later on determining what the size of the system is that you will need.  Now that you have your hand written list of items, you are ready to start putting the data into the excel spreadsheet – the “usage calculator OGS”.

Group all lights of the same kind and “watt rating” together and add the total number of lights to your list.  If  you have lights of the same size that you use for different times during the day, simply add more lines to the spreadsheet to add these items.  An estimate of average use is however good enough when it comes to lights.  Don’t stress too much over it.  Go down the spreadsheet and add all the other items to your list.  When you get to an item that is not on the spreadsheet, insert a line, and then add the item with the relevant “watt rating” data and add the time that you normally sue the item for during a normal day.

If you add lines to the spreadsheet, remember to copy the appropriate formula from the line above into the newly created cell in the “totals” column.  In the end, all totals will still add up at the bottom of the list.

Figuring out how much power each item uses

To figure out how much power each item uses, you need to check the “rating plate” or “name plate” of each item.  This is usually a sticker, or a small aluminium plate that is attached to the back of the item.  Here are some examples of some of the items in our home:


Example 1


Example 2


Example 3

In most cases, you will see a reference on the “rating plate” indicating the maximum power that the item will consume.  This is indicated in watts, it is a number, followed by a “w”.  In the picture above – Example 1 – there is a line on the “rating plate” that says “230V – 50Hz 180W”.  This clearly indicates to you that this item uses a maximum of 180 watts when you use it.  Now, most items will not consume the maximum power all the time, but for our calculation purposes, this is the number we are interested in.

In some cases, like in the second example above, you will have to calculate the power rating of the item because the manufacturer only gives you the voltage, and the current (amperage) that the item uses.  In the case of the second picture above (a “Whirlpool” box freezer), the “rating plate” has a line on it that says “230V 50Hz 0.98A”.  All the information that you need to work out its power rating is on this line.  All you need to do is multiply the volts with the amps.  In this case it means that you take the voltage rating of 230 volts and you then multiply it with the current rating which is 0.98 amps.  In electrical terms, the formula to calculate power is: P = I * V, or Watts = volts x amps.  This comes from Ohm’s law… but don’t worry too much about the rest, for now this is all you need to know to do what you need to do right now.  So, for our example above, the power rating for this box freezer is 225.4 watts [0.98A x 230V = 225.4 W].  We got all this information from the “rating plate” and through  this simple calculation, we now know how much power this item will consume when it is in use.

Sometimes the manufacturer will put the voltage, current and power rating on the “rating plate”.  In this case, like in the third example above, you don’t need to do any calculations, just use the power rating.  In this case it is 85W.  Add the 85 watts into the appropriate cell in the spreadsheet.

Just to also cover the last possibility of information on a “rating plate” – when you see a line with a number followed by “VA”, use this number for the power rating of the item.  The reference “VA” is called volt-ampere, which is just simply an alternating current reference to the power rating.  Again, don’t worry too much about the differences between watts and volt-amperes… it gets pretty hairy and it really doesn’t impact what we are busy with here!  I will explain these technical differences to clarify any questions in a post way into the future!

If all else fails and you can’t figure out what the power rating is of a specific item, please look at the average “power rating” in the spreadsheet that I attached to this post and use a power rating in the middle of the range.  It won’t give us exact figures, but it will help get us unstuck in order to move on and determine the size of the required system.

Simplified summary

All you need to do to determine how much energy you use is the following:

  • Make a handwritten list of all the items in your home that uses electricity (Include outbuildings, the garage, and remember to add the hot water system if you have one!) – walk through your home room by room and complete the list on a piece of scrap paper
  • Get all the relevant information from “rating plates” or “name plates” on the items
  • If all else fails, use the “middle of the road” option that you can get from the list of “power ratings” of typical items used in our homes in the spreadsheet [The column to the right of each item contains this “power rating” range]
  • Take all the data and add it to the spreadsheet that I provided with this blogpost – let the spreadsheet do the calculations for you
  • Look at the “bottom line” – this will indicate the total usage [both peak demand and total watt hours per day]; this is where we will start with one of the next steps to size the system that you will need

OK, I think that is about it that you will need to determine how much power you use in your home.  If anything is unclear or you have any questions, please respond to my email with your questions or just simply comment in the space provided below the blogpost.  Next week we will figure out how much sunlight you can expect on average during the year, and then we will start the calculations to get to the size of the system that you will require.  Until then…

Generating my own solar electricity – where do I start?

Living off the grid doesn’t mean that we give up on all the comforts we have gotten used to over time… Although we have to make certain lifestyle choices, having electricity is probably one element that most people going off the grid want to keep on having.  Generating your own electricity by using a solar system may at first look like a daunting task, but once you understand the basics, it is actually pretty simple.

In the previous blogpost I described what to do as a first step to reduce your electricity demand, or the amount of electricity that you will need from such a system.  In this article I would like to describe a basic solar system, in the most simple terms possible.  Next week, I would like to invite you to start doing some homework of your own, to figure out how much electricity you will need for your specific lifestyle in your home.

A basic solar electricity system looks like this:


The energy of the sun is collected through the solar panels and converted into DC electricity, regulated through a maximum power point tracker (or other regulator), and then used to charge your battery bank.  The energy from the battery bank is then fed into the unit called an “inverter” which will take the DC electricity from the batteries, and convert it (“invert it”) into AC electricity, which is then used to power up your home.

When you experience long periods of overcast weather, you may need to also connect an optional back-up generator to charge your battery bank when there isn’t enough sunlight hours available to do the job for you.  One can get real technical about this but I don’t think it is necessary – this explains the basic workings of the system.

Solar panels:

This is the part of your system that collects and converts energy from the sun into direct current electricity.  This electricity is fed into a charge controller or regulator, after which it can either be used directly in your home, or stored in batteries for later use.  Today, various different technologies of solar panels are available.  I will explain these technologies, the differences, and the pros and cons of each in a future blogpost, but for now just know that the best approach is to buy the best quality panels that your budget will allow.  The “size” of your solar panels is measured in watts, in electrical terms.  We are not here talking about the physical size, but rather the electrical capacity of the panel.  Sizes of panels vary but for home application, you would most probably buy 250 watt to 300 watt panels, depending on the specific manufacturer.

Charge controller (“MPPT” – Maximum Power Point Tracker or other):

The purpose of this piece of equipment is to regulate and control the energy from the solar panels, to the batteries. The charge controller both protects the batteries from over-charging, and it also makes sure that the maximum available amount of energy is transferred from the panels to the batteries.  That is also where the acronym or the name for the most popular charge controllers used today comes from – MPPT, meaning Maximum Power Point Tracking.  The “size”of the MPPT is measured in Amps, which indicates the maximum current (Amps) the controller can handle from the panels to the batteries. [Example – A 60 Amp or 100 Amp MPPT will be able to handle a maximum of 60 amps or 100 amps respectively].  The only other aspect to be aware of in terms of charge controllers, is the maximum input voltage that it can handle, coming from the solar panels. [This is normally around 150V for a 100 Amp MPPT]

Battery bank:

Apart from the inverter, the batteries are most probably the most important part of your system, if you are completely off the grid, or if you plan to use electricity from your system after the sun has set.  If you have a system that is connected to the grid, then remember that batteries are not necessarily needed – it is an optional extra if you want to store energy during the day for later use.  If you are going to go off grid however, to me it makes sense that you should include some storage capacity in the form of batteries, otherwise you literally can only use the energy from the sun during daylight hours.  It kind of misses the point of the whole exercise if you don’t include at least a minimum quantity of battery storage capacity.  The “size” or capacity of batteries is measured in amp hours (Ahrs), while the specific battery will also have a voltage (normally 12 volts).  The amount of energy that you could then get from the batteries after the sun has set, is measured or calculated in watt hours or amp hours – both an indication of power available over a period of time.  It literally indicates to you  how many watts you can draw from the battery bank per hour, or for a number of hours.  We will get into more detail on this when we start doing your system design based on your specific requirements.


This piece of equipment takes the energy stored in the battery bank, which is DC electricity, and converts (inverts) it into usable AC (alternative current) electricity.  The “size” or capacity of the inverter is usually measured in kilo watt (kW), which is an indication of the maximum amount of “power” that it can convert or transfer to your home.  Another measure of size of an inverter that is regularly used is kilo volt ampere (kVA) which is also a measure of “power”.  The difference between the two is pretty technical but for now, remember the measure of kilo watt, because this will just make the calculations of your electricity needs so much easier.  An inverter is also made or designed to a specific system voltage – we typically find 12 volt, 24 volt and 48 volt inverters in the marketplace.  Depending on your specific application, this is another choice you will have to make when designing your system.  We will talk about the reasons why you would choose which voltage in a later discussion.  For now, know that the most popular voltage chosen is a 48 volt inverter.

Optional generator:

The last component in the picture above is the optional generator, which is used to assist with charging the batteries when overcast or rainy weather happens.  This is optional indeed, and if you installed enough solar panel capacity to charge the batteries, the generator will really be used only in extreme circumstances.  Remember that the energy generated by a generator is much more expensive than most other forms of electricity and it should  therefor be used sparingly.

Each of the above parts of the system has to be sized correctly to make sure everything works well together…  The size of each of these will depend on your specific needs, and that is what we will get into in the next post.

Simplified summary:

A basic solar electricity system consists of the following components:

  • A solar array
  • A charge controller (MPPT or other)
  • A battery bank
  • An inverter
  • An optional generator

The sizes of each of the components will be determined by how much electricity you need for your specific application.

Guys, that is about as simple as I can explain a basic solar electricity system.  I hope it has been of value to you and that you can see that it really is not that complicated at all.  Once you understand the basics of what is required for the system to work, it will be much easier for you to connect with a potential supplier of this equipment to you.  You will most definitely know what you are talking about and it will allow you to understand what is being proposed to you as a solution for your specific needs.  Next week I will help you to start the process of determining how much electricity you will need to maintain those comforts that you just don’t want to live without.  Until then!